By default, the message bodies are not indexed and cannot be searched. You can add a helper class to implement the search capability in the Message Bank.

See documentation

There is a difference in behaviour for async vs sync messages if I read the documentation. In my case the messages are synchronous, so this should not apply. 

Any updates on this issue @Vic Sun & @Scott Roth? Was it solved when the setting was done in the right namespace?

Thanks,

Marcel